Chat capability + Mattermost self-host + stage-shaped defaults

New capability area Chat with Mattermost self-host: when self-host fits by stage, VPS vs shared-hosting matrix, Linear/Composio/webhook paths, file limits, R2/B2 + CDN offload.

Reframed Defaults by org stage as phase sections (solo → product shipping → small team → compliance) so bootstrap steps are skipped until maturity demands them; kept a compact summary matrix.

Stack map, hub index, and nav list Chat alongside Email · Scheduling · Design · Vault · Org infra.